Power Line Communications (PLC) is a rapidly growing market. PLC is attractive because it uses existing power lines that are ubiquitous in homes and businesses around the world. PLC products have proven to be very successful for in-home data distribution. However, other market segments such as Smart Grid (power utilities controlling power distribution infrastructure and major electrical loads) and Access BPL (use of power lines to provide high speed internet access to customers not served by cable or DSL) call for broadband data to be transmitted to homes and businesses over outdoor power lines.
Due to limited bandwidth (e.g., 2-80 MHz) and regulatory limits on radio frequency emissions, digital transmissions over power line have limited range, typically 1-2 km. In order to propagate signals over longer distances on a power line, digital repeaters are mounted on pole tops at distances corresponding to the range limitations of the power line. Reaching customers located at the extreme end of a power line can requires as many as 25 hops.
Transmissions requiring several hops consume a relatively large amount of available bandwidth—thereby reducing the available bandwidth not only for customers at the extreme end of the power line, but also for other customers sharing that particular line. Additionally, signals transmitted by a repeater typically propagate in both directions, “upstream” toward the head end and “downstream” toward remote customers. In many cases, there is only a need to send the data in one given direction, depending on the relative location of the source and destination stations. Sending data in the unwanted direction may result in interference to other data transmissions on the power line. This reduces the throughput level that would otherwise be possible. Thus, spatial reuse of the transmission line is limited, which results in a waste of valuable bandwidth in such cases.
Limited propagation distance on power lines also gives rise to another issue—the “hidden node” problem. PLC devices typically share the medium via a Carrier Sense Multiple Access—Collision Avoidance (CSMA-CA) mechanism. This is essentially a listen-before-talk scheme. If the medium is busy, a station will wait until the medium is idle before sending any queued data. The hidden node problem arises when a repeater is receiving information from a first transceiver, while a second transceiver which is beyond the range of the signal from the first transceiver begins transmission before the completion of the packet from the first transceiver. In this case, a collision may occur and both packets can be lost. This can be a serious problem, particularly at peak utilization times. These collisions most often occur because the second transceiver is physically located further from the first transmitter than the receiving repeater. In the majority of cases, the signal from the first and second transceivers arrive from different directions.